This is not the textbook wing flutter. It is initiated by bumpy conditions, and when the wing flexes upwards the flapperon hinge moves up which causes the the flapperon actuation leaver to move down and the flapperon moves up and reduces the lift on the wing causing the wing to flex down and the same thig happens in the oposite direction and so-on. If you have excesive play in the the control linkage or in the flapperon hinge support bearing (the flat plastic support under the hinge at the wingroot and the top part in the turtledeck).
I had a similar experience, I had eliminated all the freeplay at all other joints and only when I replaced the plastic bearing it was eliminated.